WASHINGTON — Unlike in the last election cycle, Sen. Jim DeMint of South Carolina is maintaining a low profile as he appears to fulfill a promise to Senate Republican leaders that he won't oppose any sitting GOP senators through his Senate Conservatives Fund.

But Senate Republican aides and campaign-finance experts say DeMint has reneged on that promise by using a stealth funding mechanism to target Sen. Dick Lugar, an Indiana Republican who's been in office since 1977 but now is facing a stiff tea party-backed primary challenge.

Team DeMint, the South Carolina Republican's campaign political action committee, last month transferred $500,000 to Club for Growth, an influential free market advocacy group based in Washington with longstanding close ties to DeMint.

Club for Growth last week started running a hard-hitting TV ad in Indiana, accusing Lugar of having backed tax increases, along with federal bailouts of Wall Street bankers and mortgage giant Fannie Mae.

"Call Dick Lugar," the ad ends. "Tell him — no more tax hikes and no more debt."

Lugar recently voted against two top DeMint initiatives: codifying in law the current moratorium on spending earmarks and preventing U.S. funds from helping to bail out debt-laden European governments.

The anti-earmark amendment, defeated by a 59-40 Senate vote Feb. 2, was crafted by Republican Sen. Pat Toomey of Pennsylvania, the former Club for Growth head whom DeMint helped elect to the Senate in 2010 with $304,000 in donations from his Senate Conservatives Fund.

In its analysis of 36 key votes dating to 2003, DeMint's fund ranks only three of the other 46 GOP senators — Susan Collins and Olympia Snowe of Maine, and Lisa Murkowski of Alaska — as more liberal than Lugar.

Jennifer Duffy, a Senate analyst with the widely read Cook Political Report, said DeMint's highly unusual transfer of such a large sum of money from his re-election PAC to a group that's made Lugar its top Republican target this year received wide notice in GOP Senate offices.

"The fact that DeMint used his campaign account to take this action looks far more personal and drives the perception that he has gone back on his promise not to go after his Republican colleagues," Duffy told McClatchy.

Two senior Senate Republican aides, who requested that they not be named in order to provide candid views, gave similar accounts.

"It wasn't lost on anyone in the (Republican) conference," said one aide close to Senate GOP leaders.

One reason DeMint's move stands out is that Lugar and Sen. Orrin Hatch of Utah are the two longest-serving GOP senators, both first elected in 1976.

Barney Keller, a Club for Growth spokesman, declined to say whether DeMint's donation to the group was used to help fund the anti-Lugar ad.

"Senator DeMint is a champion of economic freedom, and we're honored to have his support," Keller said.

"I don't comment on our ad buys or on why individuals make contributions to the Club for Growth, so I would refer you to Senator DeMint's office," he said.

DeMint didn't respond to questions about Lugar, noting only that he is following the directions from Senate Republican leaders.

"Our leadership asked me not to get involved with the races of incumbents," DeMint said. "Since we have 23 (incumbent Senate) Democrats up (for re-election) this cycle, that made perfect sense to me. My focus is on supporting four, five, six solid conservatives in (GOP) primaries where there's not an incumbent."

DeMint's Senate Conservatives Fund has given a total of $2 million to Republican challengers Josh Mandel, who won Ohio's March 6 primary, Ted Cruz of Texas, Don Stenberg of Nebraska and Mark Neumann of Wisconsin.